In the manufacture of integrated circuits a frequently-used sequence of processing steps is as follows: depositing (or growing) a dielectric (or metal/polysilicon) layer on a wafer; coating a thin film of photoresist; projecting a mask pattern onto the photoresist multiple times using a stepper (photolithography system); and then developing the exposed photoresist to create a selective pattern of exposed and unexposed regions. Then the masked layer (dielectric, metal, or polysilicon) is etched or exposed to dopants, or both.
This process is repeated many times successively on the wafers under process until a complete circuit is formed. Because overlay tolerances for advanced circuit designs are on the order of .+-.0.25 .mu.m over 100 to 150 mm diameter wafers, element-by-element accuracy is crucial. In view of the required precision, and the length and complexity of the manufacturing cycle, accurate and rapid overlay calibration of the stepper's exposure control mechanism (e.g. exposure energy and focus) is critical to the productivity of the manufacturing line. Furthermore, as the microelectronics industry strives to achieve smaller device design geometries, control of linewidth or critical dimensions, has become increasingly important.
Each time a layer is set up for processing by a photolithography system, the control parameters of the photolithography system must be adjusted for a varieties of material and thickness variations in order to produce patterns within required tolerances. Control parameters include exposure energy and focus, and sometimes numerical aperture, partial coherence, and overlay distortion. Adjustment of the control parameters during the processing of semiconductor wafers requires inspection of mask patterns in photoresist to provide information for the manipulation of the control parameters of the photolithography system.
Typically, measurement and reference strategies include combinations and permutations of off-line inspection methods using optical verniers, electrical test structures, and online metrology using the local alignment and stage positioning capabilities of the system under test. These methods are slow and tedious and require lengthy interruptions in the use of very expensive stepper systems.
Accordingly, there is a need for a rapid and reproducible photolithography control method. Specifically, eliminating the need to remove the wafer from the photolithography system for inspection during its processing, particularly after exposure and before development, would increase production efficiency. However, prior to developing the photoresist, the photochemical changes caused by exposure are not visible using conventional bright field microscopy. Therefore, there is a need for an inspection system which provides images of the pattern of exposed undeveloped photoresist.